The ultimate goal is the understanding, concomitant to control, of the properties of tooth enamel and other hard tissue, particularly as they are determined by properties of the inorganic fraction. It is recognized that these properties, in turn, are determined by the detailed, real and non-ideal, atomic arrangements specifically including "impurities". Hence, atomic-scale structure-property relationships for the inorganic fraction of tooth enamel and related apatitic materials are being determined through the use of neutron and x-ray diffraction techniques and selected complementary techniques (e.g., infrared spectroscopy, laser Raman spectroscopy, wet chemical and thermo-chemical techniques), with both single crystal and polycrystalline specimenns. The methods of using the techniques separately and in concert to extract more information under the adverse circumstances of natural specimens is under constant review and development. The topics being studied specifically at the moment include development of improved model systems for the atomic-scale architecture of tooth enamel and its physical and biological implications, cause and implication of hydroxyl ion deficiency in tooth enamel, possible crystalline phases present in tooth enamel either separately or in solid solution with others, elucidation of the compositional and structural differences between tooth enamel and hydroxyapatite, mechanism of fluoride action and possible alternates, occurrence of the monoclinic phase of hycroxyapatite, "oxyapatite", the structural location and role of CO3 in tooth enamel, structural location and role of various elements present in minor amounts in tooth enamel, atomic substitution and interactions in apatites, structures of apatite isomorphs and related compounds, structure related properties of apatites (e.g., physical, dielectric, chemical, phase stabilization and transitions, etc.), preparation and characterization of apatites, and calcium deficient apatites.